The present invention relates to a spacing and centering device, in particular for rigid pipe-in-pipe systems intended for the transport of hydrocarbons.
Rigid pipes of this type comprise two coaxial tubes, a rigid external tube or carrier pipe and a rigid internal tube or flowline, which are separated by an annular space, said spacing and centering device, or distance piece, being accommodated in said annular space in order to hold the two coaxial tubes at a distance from one another.
These pipes are intended to be installed on the ocean bed by means of a laying vessel and in particular by a method known as the “reeled pipe” technique.
Pipe-in-pipe systems are typically used for underwater production lines, in particular for the transport of hydrocarbons, and it is necessary to insulate them because the heavy oils have a tendency to solidify while cooling during their transfer from the bed toward the surface. Insulation is also necessary in order to avoid the formation of hydrates, which can appear when certain types of crude oil cool down, for example when production is interrupted.
Heat insulation is effected by filling the annular space of the pipe with a material with low heat conductivity.
Furthermore, the pipe-in-pipe systems are made so that their annular space is dry in order to improve heat insulation performance. The annular space can also be kept at atmospheric pressure so that said material is subjected neither to the hydrostatic pressure to which the carrier pipe is subjected nor to the pressure of the hydrocarbon flowing in the internal pipe. Rigid pipe-in-pipe systems thus make it possible to use a great variety of materials with low heat conductivity which afford good insulation.
Conventionally, two main techniques are used for laying submarine pipes from a laying vessel, the J-lay or S-lay method and the “reeled pipe” technique.
In the first technique, the pipe is assembled on the vessel by welding lengths of tube end to end before laying on the seabed. This technique is slow and costly and requires a team of welders on board the laying vessel.
The “reeled pipe” technique comprises a stage of assembly of the pipe on land and a stage of winding up on a reel which is then transported to the production site where it is unwound along the planned location. This laying technique makes it possible to mobilize the laying vessel for a shorter period than for the J-lay method.
In the “reeled pipe” technique, the rigid pipe is deformed plastically when it is wound up on the reel, and this plastic deformation is eliminated by a straightening mechanism when the pipe is unwound. During winding up, tension is applied to the pipe in order to shape the external pipe to the reel. The external pipe is thus curved by contact with the reel or with the pipe layers already wound up.
During the winding-up and unwinding operations, the flexion forces are transferred from the carrier pipe to the flowline by the distance pieces or annular walls arranged along the pipe in the annular space. During unwinding, the pipe is straightened, the external pipe being drawn through straightening means, such as opposite rollers or an assembly of straightening tensioners (well known to the person skilled in the art). The straightening forces are applied to the exterior of the carrier pipe and are transferred to the flowline solely by means of the spacing and centering devices.
As the distribution of these devices is discontinuous along the pipe-in-pipe, straightening of the flowline takes place locally and is therefore not complete. The flowline thus has residual curves after straightening which depend on the spacing of the distance pieces in the annular space. The consequence of the resultant relative displacement of the flowline in the carrier pipe is local reductions in the annular space present between the two tubes, which leads to potential compression of the material with low heat conductivity which insulates the pipe.
With the application of the “reeled pipe” techniques to pipe-in-pipe systems, continuous lengths of insulating material must be separated by distance pieces and the spacing of the distance pieces must be sufficiently small for it to be possible to transmit the flexion forces from the carrier pipe to the flowline and for local reductions in the annular space between the two tubes to be avoided.
When the heat insulating material is of the half-shell type, the local compression of the insulating material leads to a reduction in the insulating properties and overall to a reduction in the insulating properties of the pipe. In order to compensate for this loss, it is necessary to add distance pieces and to move them closer to one another in order to prevent compression of the insulating material.
However, adding extra distance pieces, the heat insulation capacity of which is low in relation to the insulating materials used, necessarily reduces the total quantity of heat insulating material and consequently the heat insulation of the pipe.
It is also possible to modify the form of the pipe by increasing the diameter of the carrier pipe and/or reducing the diameter of the flowline in order that the quantities of insulating material can be increased, but such a solution gives rise to other problems, in particular that of modifying the diameters of common standard tubes, which increases costs and/or reduces the capacities of the pipe.
It is therefore an object of the present invention to provide a rigid pipe-in-pipe having increased insulation properties which is capable of being wound up and unwound during laying by means of the usual straightening techniques.